This invention relates in general to image scanner apparatus for producing an electrical signal representative of a scanned image. More particularly, this invention relates to an image scanner having a highly efficient fluorescent radiation collector for collecting radiation emitted, reflected, or transmitted from an image scanner by a beam of radiation.
Information contained in documents or film is frequently converted to electrical signals in order to store, process, or transmit the information. Typically, the information is scanned by means of a beam of light produced by a flying spot scanner or a laser scanner. Radiation, which is emitted, transmitted, or reflected as a result of scanning, is collected and sensed by means of a radiation sensor which produces a signal representative of the scanned information. Thus, for example, the image contained in a medical X-ray film may be converted to a digital signal by means of a laser scanner system. In such a system, a laser beam is swept across the film by means of a rotating polygon mirror, while the film is moved at a constant velocity past the scanning laser beam. The laser radiation transmitted from the film is collected by a radiation collector in communication with a radiation sensor. The sensor produces a signal which is digitized and stored in a storage system, such as magnetic tape or optical disk.
In general, in such an image scanner, either an integrating tube or fiber optic bundle is used to collect radiation transmitted from the film. The collected radiation is detected by photomultiplier tubes or photodiodes. For diffuse density measurement in X-ray film scanners, it is desirable to gather both specularly transmitted radiation and scattered radiation from the film. Known light integrating tubes have low radiation collection efficiency and radiation collected along the scan line is generally non uniform. Similarly, a fiber optic bundle radiation collector has a collection efficiency which is limited by the numerical aperture of the fibers and field non-uniformity results from non uniformities in the fiber bundle.
In order to minimize these problems, fluorescent light collectors have been proposed to collect emitted or reflected light in image scanning apparatus. Thus, for example, U.S. Pat. No. 4,371,897, issued Feb. 1, 1983, patentee Kramer, discloses a fluorescent light collector for collecting light reflected from an information bearing surface. The light collector includes a generally cylindrical rod containing a fluorescent dye dispersed throughout a generally transparent medium. A photosensor is located at one end of the rod. Reflected light, incident on the light collector, is absorbed by the fluorescent dye which emits radiation at another wavelength. The emitted radiation is conducted by internal reflection in the light rod to the photosensor where it is converted to an electrical signal. The fluorescent light collector disclosed in Kramer is disadvantageous because both light collection efficiency and uniformity are adversely effected, (1) by the loss or leakage of light which is scattered at less than the critical total internal reflection angle of the rod (leaky mode light); and (2) by the surface scattering effects of contaminants or scratches on the exterior surfaces of the collector. As a result, the Kramer fluorescent light collector has low collection efficiency and end to end non uniformity of light collection (about + or - 28% without the use of a filter or apodization of the jacket enclosing the collector).
U.S. Pat. No. 4,298,802, issued Nov. 3, 1981, patentees Quella et al., discloses a fluorescent light collector comprising a transparent body which acts as a light trap. The body contains at least two different groups of fluorescent particles. Each group has a light absorption range and light emitting range which are different from the other groups. Although the disclosed fluorescent light collector increases the efficiency of broad band absorption of light by the use of multiple fluorescent dyes in a single light collector body, light collection efficiency is limited. This limited efficiency results from leaking light which is reflected at angles less than the critical total internal reflection angle. Moreover, surface scattering effects resulting from contaminants or scratches on the exterior surface of the collector are disadvantageous.
U.S. Pat. No. 4,149,902, issued Apr. 17, 1979, patentees Mauer et al., discloses apparatus for concentrating radiant energy (such as solar radiation) which is collected over a large area to a smaller area. The apparatus includes a massive slab of transparent material having three layers of fluorescent dye material bonded to a maJor surface thereof. The outer layers contain a different type of dye then the middle layer. The layers are optically contacted with each other by using an index matching fluid. The first dye contained in the middle layer absorbs a shorter wavelength of light than the second fluorescent dye contained in the outer layers. Thus, radiation absorbed by the first fluorescent dye of the middle layer is converted to emitted radiation at a longer wavelength which is absorbed by the second fluorescent dye contained in the outer layers. This structure improves the efficiency of broad band absorption of the solar spectrum. The multiple layers of fluorescent material act as a single light pipe structure since the layers are optically contacted and have the same refractive index. Consequently, emitted light falling outside of the angle of internal reflection of the layers will leak out of the multilayer structure and remain uncollected, thus, reducing the efficiency of the disclosed structure. Moreover, the massiveness of the optically transparent slab, although suitable for solar collection, is not suitable for collecting radiation in an image scanner.